The production of alkylene oxides by oxidation of an appropriate alkene in the presence of a suitable catalyst is well known.
Brian J. Ozero, Handbook of Chemicals Production Processes, edited by Robert Meyers, McGraw Hill Book Co. (1986) at Chapter 1.5, discusses cyclic processes using both oxygen and air as oxidant for the production of ethylene oxide (EO) from ethylene. In these processes, the alkene is oxidized in a multitubular catalytic reactor in vapor phase. The reactor off gases are cooled and scrubbed with water in an absorber to recover ethylene oxide which is sent to the recovery section for further purification.
In the oxygen-based process described by Ozero, the scrubber off gases are divided into three parts which are: (i) recycled to the reactor, (ii) vented and (iii) sent to a separator for carbon dioxide removal and recycle of the remaining hydrocarbons. This process suffers from several disadvantages. For example, the oxygen-based ethylene oxide process requires a separate carbon dioxide removal unit and a purge to remove argon to prevent its accumulation.
In the air-based process described by Ozero, the scrubber off gases are sent to a second reactor, which is the purge reactor, where additional unreacted ethylene is reacted using a higher air to ethylene ratio, foregoing some EO selectivity. The reactor off gases are again passed through another water scrubber to recover EO produced. It is known that the volume of hydrocarbons purged, when utilizing air as a source of oxygen, requires that the purge scrubber off gases be incinerated to remove any remaining hydrocarbons in order to meet environmental regulations. In this air-based process, an additional purge oxidation reactor, a water scrubber, and an effluent incinerator are required, as well as a greater volume of catalyst. Also, both this and the oxygen-based process use the expensive ethylene as the raw material. The processes described by Ozero are limited to either pure oxygen or air.
Khoobiar et al., U.S. Pat. No. 4,609,502, discloses a cyclic process for producing acrylonitrile using propane as a starting material. This process differs from the production of oxides by the presence of ammonia and the choice of catalyst. In the process disclosed by Khoobiar et al., the alkane is initially dehydrogenated catalytically in the presence of steam to form propylene. This is in contrast to most conventional dehydrogenation processes which avoid steam primarily due to the costs involved. After ammoxidation, the effluent is quenched to remove the desired product, and the off-gases, including propylene and propane, are sent to an oxidation reactor to remove oxygen by selective reaction with hydrogen to form water vapor. The gas mixture exiting the selective oxidation reactor includes substantial amounts of methane, ethane and ethylene, which are byproducts of dehydrogenation, and unreacted propylene and propane, in addition to carbon oxides. A sufficient portion of this gas mixture is purged to remove the net production of carbon oxides and light hydrocarbons.
Optionally, this gas mixture is split and a portion sent to a separator which removes only carbon dioxide. A portion of the effluent from the separator is purged to remove light hydrocarbons. The nonpurged portion is mixed with the remainder of the oxidation reactor effluent, fresh propane, and steam, if necessary. This mixture is sent to the dehydrogenator where the propane is converted to propylene. Another option is to cool and liquify the C.sub.3 hydrocarbons from the oxidation reactor, and then vaporize the hydrocarbons prior to recycle.
In the process disclosed by Khoobiar et al., there is no practical way to remove byproducts of propane dehydrogenation, such as methane, ethane, ethylene and the like, thereby preventing their accumulation in the system, other than by removing them in a purge stream. The removal of these gases in a purge stream will likewise result in a loss of the circulating propane and propylene, thus causing a significant decrease in the overall yield of propylene to acrylonitrile. While, as mentioned above, proPane and propylene can be recovered from the stream prior to venting, this requires additional refrigeration apparatus to cool and liquify the propylene and propane. The separated C.sub.3 hydrocarbons must be vaporized prior to recycle. These operations add to the capital and power requirements of the process.
Another disadvantage of the Khoobiar et al. process stems from the use of the selective oxidation reactor to treat the gaseous effluent exiting the quench tower. This quench effluent is at ambient temperature and must be heated prior to introduction into the oxidation reactor in order to promote oxygen removal. Because there is a significant amount of oxygen in the quench effluent, the heat of reaction generated in the oxidation reactor can result in excessive temperatures in the system. There are three know methods to alleviate this problem. First, the amount of oxygen entering the oxidation reactor can be reduced by other means. Second, multiple reactors can be utilized with a cooling means between each pair of reactors. Third, a portion of the oxidation reactor can be passed through a cooling means and recycled to the feed to reduce the internal temperature of the reactor. None of these measures is attractive from the viewpoint of cost and efficiency.
The oxidation reactor in the Khoobiar et al. process is operated with oxidation catalysts such as noble metals (e.g., platinum). Olefins and carbon monoxide, which are generated in the dehydrogenation reactor, are known to deactivate these catalysts, as disclosed in, Charles L. Thomas, Catalytic Processes and Proven Catalysts, Academic Press (1970) at 118-119. Accordingly, multiple oxidation reactors must be used (see Khoobiar et al. at column 4, lines 51-56) to allow for frequent regeneration of the catalyst which represents yet another addition to production costs. These consideration apply as well to the catalytic production of oxides from alkanes as contemplated herein.
It is therefore apparent that the industry is still searching for a cost effective process of converting alkanes into oxides. The process of the present invention is cost effective and substantially reduces or eliminates disadvantages of the aforementioned systems. Moreover, in comparison to conventional processes, the thermal requirements of the present invention process are markedly reduced.